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AUGUST 2022 InTouch | 19 Charcot-Marie-Tooth (CMT) disease, named after three doctors who each described the condition in the 1800s, is a group of similar hereditary conditions which cause the vast majority of inherited sensorimotor neuropathy. A study in 2017, led by Dr Alice Theadom (AUT), and Miriam Rodrigues (CBR Neurogenetic Clinic) showed that it is diagnosed in a minimum of 1 in ~4000 people in New Zealand, similar to the rate seen in the rest of the world. CMT involves the peripheral nerves of the arms and legs, affecting the transmission of sensory information from the feet and hands to the brain, and of motor signals from the brain to the muscles. Impaired nerve function, foot deformities and muscle wasting, contribute to problems with balance, difficulty walking and an increased risk of falls and fall-related injuries. In the 1960s it was discovered that some forms of CMT are due to abnormalities of the nerve itself, and some are due to abnormalities of the protein sheath which surrounds and protects the nerve and markedly speeds up transmission of electrical impulses along the nerve. This protein is called myelin. CMT1 is the name given to diseases of the myelin and CMT2 is the name given to diseases of the nerves themselves. In the 1990s, many of the genes which when mutated cause CMT, were discovered. By far the commonest genetic cause of CMT was found to be due to disruptions in the timing and transmission of auditory nerve responses. There is very little known about how the vestibular system is involved in CMT: how it relates to auditory involvement nor how it progresses compared to the other aspects of the condition. Vestibular audiologist Dr Rachael Taylor and neurologist Associate Professor Richard Roxburgh have set out to answer these questions. “We may find that the vestibular assessments are an easy and non-invasive way to follow the disease course. A RESEARCH The CBR Neurogenetic Clinic lead a study on the vestibular system for CMT1A. How is hearing and balance affected in Charcot-Marie-Tooth disease? 3D illustration showing a close up view of the inner ear hair cell in the vestibular system. a duplication of the peripheral myelin protein 22 (PMP22) gene. This accounts for half of all CMT and is called CMT1A. Cranial nerves are also“peripheral nerves”, and may also be affected by Charcot- Marie-Tooth disease. The eighth cranial nerve is called the vestibulocochlear nerve because it carries fibres from both the hearing organ (the cochlea) and from the balance organs (vestibular). The association between CMT and progressive sensorineural hearing loss in some people is well established but very little is known about CMT and the vestibular system. In 1996 it was shown that, in contrast to the common mechanism of hearing loss (e.g. due to noise damage) which involves loss of inner ear (cochlear) hair cells, hearing loss in CMT is most likely Continued ...